[slowmover]

Need to take it over to Hendersons Lineup. Mechanical rolling resistance issues plus steering/handling upgrades are their forte. National reputation. It's the biggest impediment to repeatable FE results at any step of the way.

A set of Doug Thorley Tri-Y headers and non-restrictive single 3.0" mandrel bent exhaust with resonator and chambered muffler. Engine fuel delivery needs to be refined with an O2 sensor on chassis dyno. Stepping up to even simplest TBI would be a benefit (but work with source as RVs have different fuel demand than cars. ) MSD distributor as its far easier to adjust total and initial timing.

A518 trans sounds good. I'd go with Bridgestone Duravis 500 tires. Will balance up with almost no weight and last longer than anything else. Dana 70 rear axle.

See threads by Orbywan. A boat tail would be singular plus effective.

I didn't follow reasoning, but that Streamline TT a better low cost and long term bet than this rattletrap. Wood framing?

An early 70's CAYO Motorvator (Avion) or Silver Streak Class C is a better choice. In which case leave aero alone except air dam, etc. Focus on RR mech issues and upgrade drivetrain. These are far better built. Past plumbing breaks, water damage unlikely.

These and yours are on either the CB300 or MB300 chassis, if memory serves. Traveling with full propane and fresh water is SOP. There won't be any FE gain by doing otherwise, plus they add stability and full practicality. An RV is an RV BECAUSE it has its own heating and power source (propane) plus an excess of water. There's never enough of either. The "benefit" of the coach electric is far less. In fact, it's a want and not a need.

Wall interior water damage is likely on almost all conventional RVs of gen years or more. Pick up a moisture meter with probe and start investigating floor. A long thread on WOODALLS re Dodge motorhomes has been running for years.

What is your projected percentage improvement goal?